There are known power conversion apparatuses designed to obtain an output voltage of a desired form from an input voltage. Such power conversion apparatuses compare a voltage (hereinafter referred to as a “command voltage”) proportional in magnitude to an output voltage of a desired form with a voltage (hereinafter referred to as a “carrier wave”) having a certain output waveform, and output to a switching element, such as a transistor, a control signal for controlling the on or off operation of the switching element according to the comparison result.
The on or off operation of the switching element is performed based on the control signal to obtain the output voltage of the desired form.
However, when the waveform of the carrier wave is constant over time, that is, when the carrier wave frequency is constant, harmonic noise corresponding to the carrier wave frequency occurs and has an adverse effect on apparatuses having the power conversion apparatuses or peripheral electronic apparatuses.
One known apparatus in which harmonic noise is reduced is a stepping motor control apparatus taught in Japanese Unexamined Patent Application Publication No. 7-99795. Therein, the stepping motor control apparatus controls the opening and closing of a switching element based on a result of comparing in magnitude between a command voltage and a carrier wave whose carrier wave frequency is modulated into a sinusoidal-wave shape. That is, the carrier wave frequency is changed to generate a sinusoidal wave. The frequency by which the carrier wave frequency is changed serves as a modulation frequency. The stepping motor control apparatus further controls the magnitude and direction of a current to be applied from a power supply to a motor to realize a desired rotational force of the motor.